In electrostatography, an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with charged toner particles. If desired, the toner image can then be transferred to a final support material or receiver such as a web or sheet of paper and affixed thereto to form a permanent record of the original.
Historically, transfer of the toner images between support surfaces in electrostatographic applications has been accomplished via electrostatic induction using a corotron or other corona generating device. In corona induced transfer systems, the final support sheet is placed in direct contact with the toner image while the image is supported on the photoconductive surface. Transfer is induced by spraying the back of the support sheet with a corona discharge having a polarity opposite that of the toner particles, thereby electrostatically transferring the toner particles to the sheet. Exemplary corotron ion emission transfer systems are disclosed in U.S. Pat. Nos. 2,807,233 and 2,836,725.
The corotron system is relatively simple. The charges deposited electrostatically tack the final support material, such as copy paper, to the original toner support, such as the photoconducter, in addition to creating the desired electrical field affecting transfer of the toner to the paper. However, the strong attraction between the paper and the original toner support makes it mechanically difficult to separate the two supports. Further, although such a system has been proven to be useful for transferring a single toner image to a final support sheet, i.e., an image that is created by means of a single exposure and developing step, corona induced transfer does not lend itself readily for use in systems where a multiplicity of toner images must be sequentially transferred to a single support sheet as exemplified by many electrostatographic color and duplexing systems.
More recently, transfer of developed images from the photoconductor to the final support material has been attempted with the aid of a biased transfer member, such as a biased transfer roll or roller, as a means of controlling the forces acting on the toner during transfer and of avoiding the severe tacking problems encountered with the use of the corona induction system. A bias transfer member is a member for electrically cooperating with a conductive support surface to attract electrically charged particles from the support surface towards the member. The first such bias transfer roll was disclosed by Fitch in U.S. Pat. No. 2,807,233, where a metal roll coated with a resilient coating having a resistivity of at least 10.sup.6 ohm cm was used as the bias transfer member. Because of the resistivity of the coating, however, the amount of bias that could be applied to the roll was limited to rather low operating values because at higher ranges the air in and about the transfer zone began to break down or ionize causing the image to degrade during transfer. Nevertheless, bias roll transfer has become the transfer method of choice currently used in electrostatographic copying systems and apparatus since the use of a bias transfer roll generally avoids the severe tacking problems which are encountered when the corona induction system is utilized.
For example, Shelffo, in U.S. Pat. No. 3,520,604, discloses a transfer roll made of a conductive rubber having a resistivity in the range of 10.sup.11 to 10.sup.16 ohm cm. Here, in order to give the roll the needed resiliency required in most practical applications, it is reported that the coating must be relatively thick. A thick coating of high resistivity, however, acts to build up a surface charge on the roll resulting in air break down in the transfer region and eventually copy degradation.
More recently, improved bias transfer members have been disclosed which reportedly have overcome many of the electrical and image degradation problems associated with some of the previous transfer techniques.
Dolcimascolo et al, for example, in U.S. Pat. No. 3,702,482, disclose a multiple layer transfer roll member for transferring xerographic images under controlled conditions. The member is capable of electrically cooperating with a conductive support surface to attract charged toner particles from the support surface towards the member or towards a transfer material such as paper positioned therebetween. The member comprises a conductive substrate for supporting a biased potential thereon, an intermediate blanket (primary layer) placed in contact with the substrate to the outer periphery of the blanket and a relatively thin outer coating (secondary layer) placed over the blanket layer having an electrical resistivity to minimize ionization of the atmosphere when the transfer member is placed in electrical cooperation with the image support surface.
Meagher, in U.S. Pat. No. 3,781,105 discloses a similar transfer member employed in conjunction with a variable electrical bias means for regulating automatically the electrical field levels at various points on the transfer member during the transfer operation and providing constant current control.
In the preferred embodiment, the transfer members disclosed in U.S. Pat. No. 3,702,482 and U.S. Pat. No. 3,781,105, consist of a roll or roller having a central biasable conductive core further having an intermediate blanket or electrically "relaxable" layer (primary layer) surrounding and in electrical contact with the core, and further having a second blanket or electrically "self-leveling" outer layer (secondary layer) surrounding and in electrical contact with the primary layer. Under operating conditions, it is desirable for optimal image transfer to maintain a relatively constant current flow of less than about 30 micro amps in the nip area between the transfer roll surface, the transfer material and the photoconductive surface from which a developed image is to be transferred. For this condition to exist at given potentials, the resistivity of the primary and secondary layers must be within critical values and preferably be relatively constant under normally anticipated extremes of operating conditions. Preferably, it has been found that the primary layer should be a resilient elastomeric material such as a polyurethane having a volume resistivity within the range of 10.sup.7 to less than 10.sup.11 ohm cm, and the secondary layer should also be a resilient material such as a polyurethane having a volume resistivity within the range of 10.sup.11 to 10.sup.15 ohm cm.
In practice, it has been found that the elastomeric polyurethane materials which are used in these transfer members, and which exhibit resistivities within the above ranges, or the resistivities of which can be adjusted or controlled to within the above ranges, are moisture sensitive such that the resistivity may vary by as much as a factor of 50 between 10% and 80% relative humidity as a function of the amount of moisture absorbed from or lost to the surrounding atmosphere. For example, in the case of the polyurethane materials which are employed as the primary layer and which have exceptionally good electrical characteristics, the volume resistivity may change from 10.sup.11 ohm cm at low moisture contents, i.e., less than about 0.1% moisture, to 10.sup.9 ohm cm at higher moisture levels, i.e., about 2.5% moisture. Other polyurethanes suitable for use as the secondary layer exhibit resistivity variations from about 10.sup.15 to 10.sup.13 ohm cm as a function of increasing moisture content. The consequent variations in resistivity due to relative humidity effects will ordinarily give rise to erratic performance of the transfer member from day to day, particularly in terms of transfer efficiency, i.e., the quality of the image transferred unless compensated for by a concomitant change in the voltages sufficient to maintain a constant nip current as disclosed by Meagher, in U.S. Pat. No. 3,781,105.
Several attempts have been made to control both the resistivity of such materials to within the critical ranges necessary for optimal image transfer and, at the same time, to reduce the moisture sensitivity of such materials to changes in relative humidity so that the resistivity of the materials remains relatively constant within the ranges required for optimal image transfer.
Chen et al, for example, in U.S. Pat. No. 4,729,925 and U.S. Pat. No. 4,742,941 disclose, as coating materials for biasable transfer members, polyurethane elastomers made from certain polyisocyanate prepolymers and polyols in which the resistivity can be maintained between 1.0.times.10.sup.9 and 1.0.times.10.sup.11 ohm cm by copolymerizing with the polyisocyanate prepolymers and polyol hardening compounds used to make the polyurethane elastomers certain polyol charge control agents formed from certain metal salts complexed with particular polyester diols such as, for example, bis[oxydiethylenebis(polycaprolactone)yl]5-sulfo-1,3-benzenedicarboxylate, methyltriphenylphosphonium salt. The polyurethane elastomers of Chen et al, however, are moisture sensitive. Reference to curve 2 in FIG. 2 of U.S. Pat. No. 4,729,925, indicates, for example, that the volume resistivity of the conductive polyurethane elastomer of Example 15 prepared from a commercial polyurethane mix and the polyol charge control agent of Example 10 therein, i.e., bis[oxydiethylenebis-(polycaprolactone)yl]-5-sulfo-1,3-benzenedicarboxylat e, methyltriphenylphosphonium salt, decreased by a factor of about 6.5 when the relative humidity changed from 25% to about 85%.
Thus, it can be seen that there still remains a need in the art for a biasable transfer member capable of electrically cooperating with a conductive support surface to attract charged toner particles towards the member or towards a transfer material such as a sheet of paper positioned between the member and the conductive support in which the resistivity not only can be controlled or adjusted to within a specific range necessary for optimal image transfer, but also one in which the resistivity is substantially insensitive to widely varying changes in relative humidity encountered during normal operating conditions such that the resistivity remains relatively constant within the range required for optimal image transfer. The present invention provides such a biasable transfer member and methods for making same.